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What's New in Kotlin 1.1

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Starting with Kotlin 1.1, the JavaScript target is no longer considered experimental. All language features are supported, and there are many new tools for integration with the front-end development environment. See below for a more detailed list of changes.

Coroutines (experimental)

The key new feature in Kotlin 1.1 is coroutines, bringing the support of async/await, yield and similar programming patterns. The key feature of Kotlin's design is that the implementation of coroutine execution is part of the libraries, not the language, so you aren't bound to any specific programming paradigm or concurrency library.

A coroutine is effectively a light-weight thread that can be suspended and resumed later. Coroutines are supported through suspending functions: a call to such a function can potentially suspend a coroutine, and to start a new coroutine we usually use an anonymous suspending functions (i.e. suspending lambdas).

Let's look at async/await which is implemented in an external library, kotlinx.coroutines:

// runs the code in the background thread pool
fun asyncOverlay() = async(CommonPool) {
    // start two async operations
    val original = asyncLoadImage("original")
    val overlay = asyncLoadImage("overlay")
    // and then apply overlay to both results
    applyOverlay(original.await(), overlay.await())

// launches new coroutine in UI context
launch(UI) {
    // wait for async overlay to complete
    val image = asyncOverlay().await()
    // and then show it in UI

Here, async { ... } starts a coroutine and, when we use await(), the execution of the coroutine is suspended while the operation being awaited is executed, and is resumed (possibly on a different thread) when the operation being awaited completes.

The standard library uses coroutines to support lazily generated sequences with yield and yieldAll functions. In such a sequence, the block of code that returns sequence elements is suspended after each element has been retrieved, and resumed when the next element is requested. Here's an example:

import kotlin.coroutines.experimental.*

fun main(args: Array<String>) {
  val seq = buildSequence {
      for (i in 1..5) {
          // yield a square of i
          yield(i * i)
      // yield a range
  // print the sequence

Run the code above to see the result. Feel free to edit it and run again!

For more information, please refer to the coroutine documentation and tutorial.

Note that coroutines are currently considered an experimental feature, meaning that the Kotlin team is not committing to supporting the backwards compatibility of this feature after the final 1.1 release.

Other Language Features

Type aliases

A type alias allows you to define an alternative name for an existing type. This is most useful for generic types such as collections, as well as for function types. Here is an example:

typealias OscarWinners = Map<String, String>

fun countLaLaLand(oscarWinners: OscarWinners) =
        oscarWinners.count { it.value.contains("La La Land") }

// Note that the type names (initial and the type alias) are interchangeable:
fun checkLaLaLandIsTheBestMovie(oscarWinners: Map<String, String>) =
        oscarWinners["Best picture"] == "La La Land"

fun oscarWinners(): OscarWinners {
    return mapOf(
            "Best song" to "City of Stars (La La Land)",
            "Best actress" to "Emma Stone (La La Land)",
            "Best picture" to "Moonlight" /* ... */)

fun main(args: Array<String>) {
    val oscarWinners = oscarWinners()

    val laLaLandAwards = countLaLaLand(oscarWinners)
    println("LaLaLandAwards = $laLaLandAwards (in our small example), but actually it's 6.")

    val laLaLandIsTheBestMovie = checkLaLaLandIsTheBestMovie(oscarWinners)
    println("LaLaLandIsTheBestMovie = $laLaLandIsTheBestMovie")

See the documentation and KEEP for more details.

Bound callable references

You can now use the :: operator to get a member reference pointing to a method or property of a specific object instance. Previously this could only be expressed with a lambda. Here's an example:

val numberRegex = "\\d+".toRegex()
val numbers = listOf("abc", "123", "456").filter(numberRegex::matches)

fun main(args: Array<String>) {
    println("Result is $numbers")

Read the documentation and KEEP for more details.

Sealed and data classes

Kotlin 1.1 removes some of the restrictions on sealed and data classes that were present in Kotlin 1.0. Now you can define subclasses of a top-level sealed class on the top level in the same file, and not just as nested classes of the sealed class. Data classes can now extend other classes. This can be used to define a hierarchy of expression classes nicely and cleanly:

sealed class Expr

data class Const(val number: Double) : Expr()
data class Sum(val e1: Expr, val e2: Expr) : Expr()
object NotANumber : Expr()

fun eval(expr: Expr): Double = when (expr) {
    is Const -> expr.number
    is Sum -> eval(expr.e1) + eval(expr.e2)
    NotANumber -> Double.NaN
val e = eval(Sum(Const(1.0), Const(2.0)))

fun main(args: Array<String>) {
    println("e is $e") // 3.0

Read the documentation or sealed class and data class KEEPs for more detail.

Destructuring in lambdas

You can now use the destructuring declaration syntax to unpack the arguments passed to a lambda. Here's an example:

fun main(args: Array<String>) {
    val map = mapOf(1 to "one", 2 to "two")
    // before
    println(map.mapValues { entry ->
        val (key, value) = entry
        "$key -> $value!"
    // now
    println(map.mapValues { (key, value) -> "$key -> $value!" })

Read the documentation and KEEP for more details.

Underscores for unused parameters

For a lambda with multiple parameters, you can use the _ character to replace the names of the parameters you don't use:

fun main(args: Array<String>) {
    val map = mapOf(1 to "one", 2 to "two")

    map.forEach { _, value -> println("$value!") }

This also works in destructuring declarations:

data class Result(val value: Any, val status: String)

fun getResult() = Result(42, "ok").also { println("getResult() returns $it") }

fun main(args: Array<String>) {
    val (_, status) = getResult()
    println("status is '$status'")

Read the KEEP for more details.

Underscores in numeric literals

Just as in Java 8, Kotlin now allows to use underscores in numeric literals to separate groups of digits:

val oneMillion = 1_000_000
val hexBytes = 0xFF_EC_DE_5E
val bytes = 0b11010010_01101001_10010100_10010010

fun main(args: Array<String>) {

Read the KEEP for more details.

Shorter syntax for properties

For properties with the getter defined as an expression body, the property type can now be omitted:

data class Person(val name: String, val age: Int) {
    val isAdult get() = age >= 20 // Property type inferred to be 'Boolean'

fun main(args: Array<String>) {
    val akari = Person("Akari", 26)
    println("$akari.isAdult = ${akari.isAdult}")

Inline property accessors

You can now mark property accessors with the inline modifier if the properties don't have a backing field. Such accessors are compiled in the same way as inline functions.

public val <T> List<T>.lastIndex: Int
    inline get() = this.size - 1

fun main(args: Array<String>) {
    val list = listOf('a', 'b')
    // the getter will be inlined
    println("Last index of $list is ${list.lastIndex}")

You can also mark the entire property as inline - then the modifier is applied to both accessors.

Read the documentation and KEEP for more details.

Local delegated properties

You can now use the delegated property syntax with local variables. One possible use is defining a lazily evaluated local variable:

import java.util.Random

fun needAnswer() = Random().nextBoolean()

fun main(args: Array<String>) {
    val answer by lazy {
        println("Calculating the answer...")
    if (needAnswer()) {                     // returns the random value
        println("The answer is $answer.")   // answer is calculated at this point
    else {
        println("Sometimes no answer is the answer...")

Read the KEEP for more details.

Interception of delegated property binding

For delegated properties, it is now possible to intercept delegate to property binding using the provideDelegate operator. For example, if we want to check the property name before binding, we can write something like this:

class ResourceLoader<T>(id: ResourceID<T>) {
    operator fun provideDelegate(thisRef: MyUI, prop: KProperty<*>): ReadOnlyProperty<MyUI, T> {
        checkProperty(thisRef, prop.name)
        ... // property creation

    private fun checkProperty(thisRef: MyUI, name: String) { ... }

fun <T> bindResource(id: ResourceID<T>): ResourceLoader<T> { ... }

class MyUI {
    val image by bindResource(ResourceID.image_id)
    val text by bindResource(ResourceID.text_id)

The provideDelegate method will be called for each property during the creation of a MyUI instance, and it can perform the necessary validation right away.

Read the documentation for more details.

Generic enum value access

It is now possible to enumerate the values of an enum class in a generic way.

enum class RGB { RED, GREEN, BLUE }

inline fun <reified T : Enum<T>> printAllValues() {
    print(enumValues<T>().joinToString { it.name })

fun main(args: Array<String>) {
    printAllValues<RGB>() // prints RED, GREEN, BLUE

Scope control for implicit receivers in DSLs

The @DslMarker annotation allows to restrict the use of receivers from outer scopes in a DSL context. Consider the canonical HTML builder example:

table {
    tr {
        td { +"Text" }

In Kotlin 1.0, code in the lambda passed to td has access to three implicit receivers: the one passed to table, to tr and to td. This allows you to call methods that make no sense in the context - for example to call tr inside td and thus to put a <tr> tag in a <td>.

In Kotlin 1.1, you can restrict that, so that only methods defined on the implicit receiver of td will be available inside the lambda passed to td. You do that by defining your annotation marked with the @DslMarker meta-annotation and applying it to the base class of the tag classes.

Read the documentation and KEEP for more details.

rem operator

The mod operator is now deprecated, and rem is used instead. See this issue for motivation.

Standard library

String to number conversions

There is a bunch of new extensions on the String class to convert it to a number without throwing an exception on invalid number: String.toIntOrNull(): Int?, String.toDoubleOrNull(): Double? etc.

val port = System.getenv("PORT")?.toIntOrNull() ?: 80

Also integer conversion functions, like Int.toString(), String.toInt(), String.toIntOrNull(), each got an overload with radix parameter, which allows to specify the base of conversion (2 to 36).


onEach is a small, but useful extension function for collections and sequences, which allows to perform some action, possibly with side-effects, on each element of the collection/sequence in a chain of operations. On iterables it behaves like forEach but also returns the iterable instance further. And on sequences it returns a wrapping sequence, which applies the given action lazily as the elements are being iterated.

        .filter { it.isFile && it.name.endsWith(".txt") }
        .onEach { println("Moving $it to $outputDir") }
        .forEach { moveFile(it, File(outputDir, it.toRelativeString(inputDir))) }

also(), takeIf() and takeUnless()

These are three general-purpose extension functions applicable to any receiver.

also is like apply: it takes the receiver, does some action on it, and returns that receiver. The difference is that in the block inside apply the receiver is available as this, while in the block inside also it's available as it (and you can give it another name if you want). This comes handy when you do not want to shadow this from the outer scope:

class Block {
    lateinit var content: String

fun Block.copy() = Block().also {
    it.content = this.content

// using 'apply' instead
fun Block.copy1() = Block().apply {
    this.content = this@copy1.content

fun main(args: Array<String>) {
    val block = Block().apply { content = "content" }
    val copy = block.copy()
    println("Testing the content was copied:")
    println(block.content == copy.content)

takeIf is like filter for a single value. It checks whether the receiver meets the predicate, and returns the receiver, if it does or null if it doesn't. Combined with an elvis-operator and early returns it allows to write constructs like:

val outDirFile = File(outputDir.path).takeIf { it.exists() } ?: return false
// do something with existing outDirFile
fun main(args: Array<String>) {
    val input = "Kotlin"
    val keyword = "in"

    val index = input.indexOf(keyword).takeIf { it >= 0 } ?: error("keyword not found")
    // do something with index of keyword in input string, given that it's found
    println("'$keyword' was found in '$input'")
    println(" ".repeat(index) + "^")

takeUnless is the same as takeIf, but it takes the inverted predicate. It returns the receiver when it doesn't meet the predicate and null otherwise. So one of the examples above could be rewritten with takeUnless as following:

val index = input.indexOf(keyword).takeUnless { it < 0 } ?: error("keyword not found")

It is also convenient to use when you have a callable reference instead of the lambda:

private fun testTakeUnless(string: String) {
    val result = string.takeUnless(String::isEmpty)

    println("string = \"$string\"; result = \"$result\"")

fun main(args: Array<String>) {


This API can be used to group a collection by key and fold each group simultaneously. For example, it can be used to count the number of words starting with each letter:

fun main(args: Array<String>) {
    val words = "one two three four five six seven eight nine ten".split(' ')
    val frequencies = words.groupingBy { it.first() }.eachCount()
    println("Counting first letters: $frequencies.")

    // The alternative way that uses 'groupBy' and 'mapValues' creates an intermediate map, 
    // while 'groupingBy' way counts on the fly.
    val groupBy = words.groupBy { it.first() }.mapValues { (_, list) -> list.size }
    println("Comparing the result with using 'groupBy': ${groupBy == frequencies}.")

Map.toMap() and Map.toMutableMap()

These functions can be used for easy copying of maps:

class ImmutablePropertyBag(map: Map<String, Any>) {
    private val mapCopy = map.toMap()


The operator plus provides a way to add key-value pair(s) to a read-only map producing a new map, however there was not a simple way to do the opposite: to remove a key from the map you have to resort to less straightforward ways to like Map.filter() or Map.filterKeys(). Now the operator minus` fills this gap. There are 4 overloads available: for removing a single key, a collection of keys, a sequence of keys and an array of keys.

fun main(args: Array<String>) {
    val map = mapOf("key" to 42)
    val emptyMap = map - "key"
    println("map: $map")
    println("emptyMap: $emptyMap")

minOf() and maxOf()

These functions can be used to find the lowest and greatest of two or three given values, where values are primitive numbers or Comparable objects. There is also an overload of each function that take an additional Comparator instance, if you want to compare objects that are not comparable themselves.

fun main(args: Array<String>) {
    val list1 = listOf("a", "b")
    val list2 = listOf("x", "y", "z")
    val minSize = minOf(list1.size, list2.size)
    val longestList = maxOf(list1, list2, compareBy { it.size })
    println("minSize = $minSize")
    println("longestList = $longestList")

Array-like List instantiation functions

Similar to the Array constructor, there are now functions that create List and MutableList instances and initialize each element by calling a lambda:

fun main(args: Array<String>) {
    val squares = List(10) { index -> index * index }
    val mutable = MutableList(10) { 0 }

    println("squares: $squares")
    println("mutable: $mutable")


This extension on Map returns an existing value corresponding to the given key or throws an exception, mentioning which key was not found. If the map was produced with withDefault, this function will return the default value instead of throwing an exception.

fun main(args: Array<String>) {

    val map = mapOf("key" to 42)
    // returns non-nullable Int value 42
    val value: Int = map.getValue("key")

    val mapWithDefault = map.withDefault { k -> k.length }
    // returns 4
    val value2 = mapWithDefault.getValue("key2")

    // map.getValue("anotherKey") // <- this will throw NoSuchElementException
    println("value is $value")
    println("value2 is $value2")

Abstract collections

These abstract classes can be used as base classes when implementing Kotlin collection classes. For implementing read-only collections there are AbstractCollection, AbstractList, AbstractSet and AbstractMap, and for mutable collections there are AbstractMutableCollection, AbstractMutableList, AbstractMutableSet and AbstractMutableMap. On JVM these abstract mutable collections inherit most of their functionality from JDK's abstract collections.

Array manipulation functions

The standard library now provides a set of functions for element-by-element operations on arrays: comparison (contentEquals and contentDeepEquals), hash code calculation (contentHashCode and contentDeepHashCode), and conversion to a string (contentToString and contentDeepToString). They're supported both for the JVM (where they act as aliases for the corresponding functions in java.util.Arrays) and for JS (where the implementation is provided in the Kotlin standard library).

fun main(args: Array<String>) {
    val array = arrayOf("a", "b", "c")
    println(array.toString())  // JVM implementation: type-and-hash gibberish
    println(array.contentToString())  // nicely formatted as list

JVM Backend

Java 8 bytecode support

Kotlin has now the option of generating Java 8 bytecode (-jvm-target 1.8 command line option or the corresponding options in Ant/Maven/Gradle). For now this doesn't change the semantics of the bytecode (in particular, default methods in interfaces and lambdas are generated exactly as in Kotlin 1.0), but we plan to make further use of this later.

Java 8 standard library support

There are now separate versions of the standard library supporting the new JDK APIs added in Java 7 and 8. If you need access to the new APIs, use kotlin-stdlib-jre7 and kotlin-stdlib-jre8 maven artifacts instead of the standard kotlin-stdlib. These artifacts are tiny extensions on top of kotlin-stdlib and they bring it to your project as a transitive dependency.

Parameter names in the bytecode

Kotlin now supports storing parameter names in the bytecode. This can be enabled using the -java-parameters command line option.

Constant inlining

The compiler now inlines values of const val properties into the locations where they are used.

Mutable closure variables

The box classes used for capturing mutable closure variables in lambdas no longer have volatile fields. This change improves performance, but can lead to new race conditions in some rare usage scenarios. If you're affected by this, you need to provide your own synchronization for accessing the variables.

javax.script support

Kotlin now integrates with the javax.script API (JSR-223). The API allows to evaluate snippets of code at runtime:

val engine = ScriptEngineManager().getEngineByExtension("kts")!!
engine.eval("val x = 3")
println(engine.eval("x + 2"))  // Prints out 5

See here for a larger example project using the API.


To prepare for Java 9 support, the extension functions and properties in the kotlin-reflect.jar library have been moved to the package kotlin.reflect.full. The names in the old package (kotlin.reflect) are deprecated and will be removed in Kotlin 1.2. Note that the core reflection interfaces (such as KClass) are part of the Kotlin standard library, not kotlin-reflect, and are not affected by the move.

JavaScript Backend

Unified standard library

A much larger part of the Kotlin standard library can now be used from code compiled to JavaScript. In particular, key classes such as collections (ArrayList, HashMap etc.), exceptions (IllegalArgumentException etc.) and a few others (StringBuilder, Comparator) are now defined under the kotlin package. On the JVM, the names are type aliases for the corresponding JDK classes, and on the JS, the classes are implemented in the Kotlin standard library.

Better code generation

JavaScript backend now generates more statically checkable code, which is friendlier to JS code processing tools, like minifiers, optimisers, linters, etc.

The external modifier

If you need to access a class implemented in JavaScript from Kotlin in a typesafe way, you can write a Kotlin declaration using the external modifier. (In Kotlin 1.0, the @native annotation was used instead.) Unlike the JVM target, the JS one permits to use external modifier with classes and properties. For example, here's how you can declare the DOM Node class:

external class Node {
    val firstChild: Node

    fun appendChild(child: Node): Node

    fun removeChild(child: Node): Node

    // etc

Improved import handling

You can now describe declarations which should be imported from JavaScript modules more precisely. If you add the @JsModule("<module-name>") annotation on an external declaration it will be properly imported to a module system (either CommonJS or AMD) during the compilation. For example, with CommonJS the declaration will be imported via require(...) function. Additionally, if you want to import a declaration either as a module or as a global JavaScript object, you can use the @JsNonModule annotation.

For example, here's how you can import JQuery into a Kotlin module:

external interface JQuery {
    fun toggle(duration: Int = definedExternally): JQuery
    fun click(handler: (Event) -> Unit): JQuery

external fun jquery(selector: String): JQuery

In this case, JQuery will be imported as a module named jquery. Alternatively, it can be used as a $-object, depending on what module system Kotlin compiler is configured to use.

You can use these declarations in your application like this:

fun main(args: Array<String>) {
    jquery(".toggle-button").click {